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A Tour of NTL: Traditional and ISO Modes  </title>
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<h1> 
<p align=center>
A Tour of NTL: Traditional and ISO Modes
</p>
</h1>

<p> <hr> <p>

<p>

As of version 4.1,
NTL can be compiled and used in one of two modes: Traditional or ISO.
<i>As of NTL version 5.4, ISO mode is the default.</i>

<p>
To revert to traditional mode, you can pass <tt>NTL_STD_CXX=off</tt>
as an argument to the configuration script
when <a href="tour-unix.html">installing NTL on a Unix or Unix-like system</a>,
which will unset the flag <tt>NTL_STD_CXX</tt> in the <tt>config.h</tt>
file.
Alternatively (and especially on non-Unix systems),
you can unset this flag  by hand by editing 
the the <tt>config.h</tt> file.
<p>



<p>
In Traditional mode, the NTL header files include the traditional
<tt>C++</tt> header files <tt>&lt;stdlib.h&gt;</tt>,
<tt>&lt;math.h&gt;</tt>, and <tt>&lt;iostream.h&gt;</tt>.
These files declare a number of names (functions, types, etc.)
in the <i>global namespace</i>.
Additionally, the NTL header files declare a number of names,
also in the global namespace.

<p>
In ISO mode, three things change:

<ol>
<li>
<b>NTL namespace:</b>
The NTL header files wrap all NTL names in a namespace, called <tt>NTL</tt>.

<p>
<li>
<b>New header files:</b>
The NTL header files include the new <tt>C++</tt> 
header files <tt>&lt;cstdlib&gt;</tt>,
<tt>&lt;cmath&gt;</tt>, and <tt>&lt;iostream&gt;</tt>.
These new header files are essentially the same as the traditional ones,
except that all the the names are declared in a namespace called 
<tt>std</tt>.

<p>
<li>
<b>Nothrow new:</b>
The NTL implementation files use the <tt>nothrow</tt> version of <tt>new</tt>.
</ol>



<p>
If your complier is not up to date, but you want some of the benefits
of Standard <tt>C++</tt>, you can set the <i>partial standard</i>
flags to get any subset of the above three changes:
<p>
<ol>
<li>
<tt>NTL_PSTD_NNS</tt>: NTL namespace
<li>
<tt>NTL_PSTD_NHF</tt>: New header files
<li>
<tt>NTL_PSTD_NTN</tt>: Nothrow new
</ol>

You can set these flags either by using the configuration script
(only on Unix-like systems), or by editing the <tt>config.h</tt> file.
For example, to just wrap NTL in a namepsace, just pass 
<tt>NTL_PSTD_NNS=on</tt>
as an argument to the configuration script
when installing NTL.
However, make sure you also turn off the <tt>NTL_STD_CXX</tt> flag;
otherwise, these have no effect.

<p>

Especially when combining NTL with other libraries, the
<tt>NTL_PSTD_NNS</tt> flag may be particularly useful
in avoiding name clashes, even if your compiler has just a
rudimentary implementation of namespaces.

<p>
NTL will remain usable in Traditional mode indefinitely,
assuming compilers maintain reasonable backward compatibilty with 
pre-standard <tt>C++</tt> conventions for header files;
however, if you want to <i>program for the future</i>, it is recommended
to use ISO mode.
The partial ISO modes are not highly recommended;
they are mainly intended as a stop-gap measure 
while we wait for decent standard-conforming <tt>C++</tt>
compilers to become available.


<p>
<h3>
A crash course on namespaces
</h3>

<p>
As already mentioned, the main difference between Traditional and ISO
mode is that in ISO mode, all names are wrapped in namespaces.
Namespaces are a feature that was introduced in the new <tt>C++</tt> standard.
One can declare names (functions, types, etc.) inside a namespace.
By default,
such names are not visible outside the namespace without explicit
qualification.

<p>
The main advantage of namespaces is that it solves the <i>namespace pollution
problem</i>:
if two libraries define the same name in two inconsistent ways,
it is very difficult, if not impossible,
to combine these two libraries in the same 
program.

<p>
The traditional way of avoiding such problems in languages like
<tt>C</tt> is for a library designer to attach a prefix specific
to that library to all names.
This works, but makes for ugly code.
The function overloading mechanism in <tt>C++</tt> eases the problem a bit,
but is still not a complete solution.

<p>

The new
namespace feature in <tt>C++</tt>
provides a reasonably complete and elegant solution to the namespace
pollution problem.
It is one of the nicest and most important recent additions to the <tt>C++</tt>
language.

<p>

Here is a simple example to illustrate namespaces.
<p>
<pre>

namespace N {
   void f(int);
   void g(int);
   int x;
}

int x;

void h()
{
   x = 1;    // the global x
   N::x = 0; // the x in namespace N
   N::f(0);  // the f in namespace N
   g(1);     // error -- g is not visible here
}

</pre>

<p>
All of this explicit qualification business
can be a bit tedious.
The easiest way to avoid this tedium is to use what is called
a <i>using directive</i>, which effectively makes
all names declared within a namespace visible in the
global scope.

Here is a variation on the previous example, with a using directive.

<p>
<pre>

namespace N {
   void f(int);
   void g(int);
   int x;
}

int x;

using namespace N;

void h()
{
   x = 1;    // error -- ambiguous: the global x or the x in namespace N?
   ::x = 1;  // the global x
   N::x = 0; // the x in namespace N
   N::f(0);  // the f in namespace N
   f(0);     // OK -- N::f(int) is visible here
   g(1);     // OK -- N::g(int) is visible here
}

</pre>

<p>
Here is another example.

<p>
<pre>

namespace N1 {
   int x;
   void f(int);
   void g(int);
}

namespace N2 {
   int x;
   int y;
   void f(double);
   void g(int);
}

using namespace N1;
using namespace N2;

void h()
{
   x = 1;     // error -- ambiguous: N1::x or N2::x?
   N1::x = 1; // OK
   N2::x = 1; // OK
   y = 1;     // OK  -- this is N2::y
   g(0);      // error -- ambiguous: N1::g(int) or N2::g(int)?
   f(0);      // OK -- N1::f(int), because it is the "best" match 
   f(0.0);    // OK  -- N2::f(double), because it is the "best" match
}

</pre>

<p>
This example illustrates the interaction between using declarations
and function overloading resolution.
If several overloaded versions of a function are visible,
it is not necessarily ambiguous: the usual overload resolution
procedure is applied, and if there is a unique "best" match,
then there is no ambiguity.

<p>

The examples presented here do not illustrate all of the
features and nuances of namespaces.
For this, you are referred to a <tt>C++</tt> book.

<p>
<h3>
Namespaces and NTL
</h3>

<p>
In ISO mode, the standard library is "wrapped" in namespace <tt>std</tt>,
and NTL is "wrapped" in namespace <tt>NTL</tt>.
Thus, the header file <tt>&lt;NTL/ZZ.h&gt;</tt> in ISO mode looks
something like this:
<pre>

namespace NTL {

   // ...

   class ZZ { /* ... */ };

   // ...

   ZZ operator+(const ZZ&amp; a, const ZZ&amp; b);
   ZZ operator*(const ZZ&amp; a, const ZZ&amp; b);

   std::istream&amp; operator>>(std::istream&amp; s, ZZ&amp; x);
   std::ostream&amp; operator<<(std::ostream&amp; s, const ZZ&amp; a);

   // ...

  
}

</pre>

Therefore, one must explicitly qualify all names, or use appropriate
using directives.
Here is how one could write the <a href="tour-ex1.html">first example</a> 
of the tour in
ISO mode.

<pre>

#include &lt;NTL/ZZ.h&gt;

int main()
{
   NTL::ZZ a, b, c; 

   std::cin &gt;&gt; a; 
   std::cin &gt;&gt; b; 
   c = (a+1)*(b+1);
   std::cout &lt;&lt; c &lt;&lt; "\n";
}

</pre>

<p>
Notice how everything is explicitly qualified.
Actually, the input/output operators <tt>&lt;&lt;</tt> and <tt>&gt;&gt;</tt>,
and the arithmetic operators <tt>+</tt> and <tt>*</tt> are not explicitly
qualified, but rather, the compiler finds them through a gimmick
called <i>Koenig Lookup</i>, which will look for functions (and operators)
declared in namespace <tt>NTL</tt>, because the type of the argument
(<tt>ZZ</tt>) is a class declared in that namespace.

<p>

Even with Koenig Lookup, explicit qualification can
be a bit tedious.
Here is the same example, this time with using directives.

<pre>

#include &lt;NTL/ZZ.h&gt;

using namespace NTL;
using namespace std;

int main()
{
   ZZ a, b, c; 

   cin &gt;&gt; a; 
   cin &gt;&gt; b; 
   c = (a+1)*(b+1);
   cout &lt;&lt; c &lt;&lt; "\n";
}

</pre>

To write NTL client code that will compile smoothly in either
Traditional or ISO mode, one simply does the following:

<pre>

#include &lt;NTL/ZZ.h&gt;

NTL_CLIENT

int main()
{
   ZZ a, b, c; 

   cin &gt;&gt; a; 
   cin &gt;&gt; b; 
   c = (a+1)*(b+1);
   cout &lt;&lt; c &lt;&lt; "\n";
}

</pre>

<p>
Here, <tt>NTL_CLIENT</tt> is a macro defined by NTL
that expands into zero, one, or two appropriate <i>using</i> directives,
depending on the settings of <tt>NTL_STD_CXX</tt>,
<tt>NTL_PSTD_NNS</tt>, and <tt>NTL_PSTD_NHF</tt>.
Alternatively, instead of using the <tt>NTL_CLIENT</tt> macro,
you can write:
<p>

<pre>
#if (defined(NTL_PSTD_NNS) || defined(NTL_STD_CXX))
   using namespace NTL;
#endif

#if (defined(NTL_PSTD_NHF) || defined(NTL_STD_CXX))
   using namespace std;
#endif
</pre>

Typically,
when writing a program that uses NTL,
you can
simply insert the <tt>NTL_CLIENT</tt> as above,
and forget about all this namespace nonsense.
However, if you are combining libraries, you may have to disambiguate
things from time to time.

<p>

The Standard <tt>C++</tt> library is huge.
If you just use <tt>&lt;iostream&gt;</tt>, you should not
have any ambiguous names.
However, there are some potential ambiguities in the STL
(Standard Template Library) part of the library.
One that I know of is the template class <tt>negate</tt>
defined in <tt>&lt;functional&gt;</tt>, which conflicts with the
NTL function <tt>negate</tt>.
With namespaces, there should be no problem, unless the client
code explicitly uses <tt>negate</tt>, in which case you will
have to explicitly qualify <tt>negate</tt> to tell the compiler
which <tt>negate</tt> you mean, either <tt>std::negate</tt>
or <tt>NTL::negate</tt>.

<p>
NTL also explicitly defines various versions of <tt>min</tt>
and <tt>max</tt> functions.
Template versions of these functions are also defined in the
standard library component <tt>&lt;algorithm&gt;</tt>.
Because of the way the function overload resolution mechanism works, 
the "right" version of <tt>min</tt> or <tt>max</tt> should always
be chosen, without any need for explicit qualification.

<p>
There may be other possible ambiguities between the standard library
and NTL, but if they arise, they are easily fixed through
explicit qualification.

<p>
<h3>
Some global names
</h3>
<p>

It is not quite true that <i>all</i> names
declared in NTL header files are wrapped in namespace NTL.
There are two classes of exceptions:
<p>
<ul>
<li>
All names that start with the prefix "<tt>NTL_</tt>"
are in fact <i>macros</i>.  
There are a number of documented and undocumented
such macros.
Note that any name with this prefix is a macro and all macros
start with this prefix.

<p>

<li>
There are also a number of undocumented names that start with the 
prefix "<tt>_ntl_</tt>".
These are not macros, but rather are names of functions, types, etc., 
that are declared in the global namespace.
Any name with this prefix is in the global namespace,
and all names in the global namespace start with this prefix.
All functions with <tt>"C"</tt> linkage have this prefix.
</ul>
<p>
Thus, NTL "owns" all names starting with "<tt>NTL_</tt>" or "<tt>_ntl_</tt>";
users of NTL should avoid names with these prefixes.

<p>
<h3>
Further technicalities
</h3>
<p>

Another thing to be aware of is that there are some small, annoying
differences between the old standard <tt>C</tt> include files
<tt>&lt;stdlib.h&gt;</tt> and <tt>&lt;math.h&gt;</tt>,
and the new <tt>C++</tt> include files 
<tt>&lt;cstdlib&gt;</tt> and <tt>&lt;cmath&gt;</tt>,
above and beyond the namespace wrapping.
Specifically, the new header files declare several overloaded versions
of some functions.
For example, in the old header files, there was one function
<pre>
   int abs(int);
</pre>
Now there are several, including:
<pre>
   int abs(int);
   long abs(long);
   float abs(float);
   double abs(double);
   long double abs(long double);
</pre>
Also, functions like <tt>log</tt> and <tt>sqrt</tt> are also overloaded.
So instead of just
<pre>
   double log(double);
</pre>
there are
<pre>
   float log(float);
   double log(double);
   long double log(long double);
</pre>

<p>
This can lead to compile-time errors in some old codes, such as:
<pre>
   double log_2 = log(2);
</pre>

<p>
With the old header files, the <tt>int</tt> value 2 would have
been converted to a <tt>double</tt>, and the function 
<pre>
   double log(double);
</pre>
would have been called.
<p>
With the new header files, the compiler would raise an error,
because the function call is now ambiguous.
<p>
Of course, the fix is trivial:
<pre>
   double log_2 = log(2.0);
</pre>
This will compile correctly with either old or new header files.

<p>
Don't you just love the ISO?


<p>
<h3>
A note on documentation
</h3>
<p>

The "<tt>.txt</tt>" files documenting NTL's modules
still reflect NTL's  Traditional mode.
There should be no confusion in interpretting the meaning in ISO mode.
Just remember: all of NTL is wrapped in namespace <tt>NTL</tt>,
and the standard library is wrapped in namespace <tt>std</tt>.


<p>
<h3>
Further changes in NTL version 4.1
</h3>
<p>

The ISO Standard for <tt>C++</tt> is not compatible with the
language defined in the second edition of Stroustrup's <tt>C++</tt> book.
This is in fact quite annoying.
Besides introducing namespaces, several modifications were made
in version 4.1 that will allow NTL to be compiled smoothly under
<i>either</i> the old or the new definition of the language
(or any reasonable approximation thereof).
These changes do not affect the (documented) NTL interface,
and so version 4.1 should be backward compatible.
<p>
Here is a summary of the other changes:
<ul>
<li>
Got rid of all <tt>friend</tt> functions.
It turns out that new <tt>C++</tt> and old <tt>C++</tt> disagree 
quite strongly about the semantics of a <tt>friend</tt> function
declaration.
In getting rid of these, I also made a number of fields public
which used to be private, but to prevent accidental misuse,
I gave them strange names (e.g., the previously
private member <tt>rep</tt> in class <tt>ZZ_p</tt>
is now the public member <tt>_ZZ_p__rep</tt>).

<p>
This change is effective in both Traditional and ISO modes.

<p>
In my view, the ISO committee really committed an act of sabotage here.
Now the <tt>friend</tt> mechanism is much more awkward than before,
which makes the use of private members more awkward,
which simply encourages programmers (like me) to avoid them altogether.

<p>

<li>
When <tt>NTL_STD_CXX</tt> or <tt>NTL_PSTD_NTN</tt> are set, 
all calls to <tt>new</tt>
have been replaced by <tt>new(std::nothrow)</tt>.

<p>
The ISO committee also committed an act of sabotage when they changed
the semantics of the memory allocation operator <tt>new</tt>.
In old <tt>C++</tt>, a memory allocation error simply returned
a null pointer; in new <tt>C++</tt> an exception is thrown.
The old semantics are available via  <tt>new(std::nothrow)</tt>.

<p>
You may of course use NTL in Traditional mode with a compiler that
implements the new semantics for <tt>new</tt>.
In this case, if the memory allocation fails, an exception will
be thrown, and assuming you don't catch it, you will simply get an
error message that is less informative than the one NTL would
have printed.
Also, your compiler may have a backward compatatibilty flag to 
use the old <tt>new</tt> semantics.

<p>

<li>
Various and sundry other small changes, such as fixing
occurrences of the
the "<tt>log(2)</tt>" problem mentioned above.

</ul>

<p>


<p>
<h3>
Standard C++ and the Real World
</h3>

<p>
The first C++ standard was set in 1998, with some
revisions in 2003.
As I write this update in 2013, I believe it is safe
to say that most compileres released in the last
few years do a pretty good job of implementing the standard.

<p>
However, a new revision to tne standard appeared in 2011.
This new revision contains many new language and library 
features.
Surely, it will be a number of years until 
support for all these new feautures will be uniformly 
available.



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